Tubular polymerization reactors and polymers made therein

ABSTRACT

Tubular reactor apparatus and processes are provided for improved polymerization including using chain transfer agents and multiple monomer feeds spaced lengthwise along the tubular reactor providing high conversions of monomer into polymer. The invention also relates to polymers made from such a tubular reactor apparatus and processes including those polymers having a low haze value, a density over 0.92 g/cm 3  and/or having terminal carbonyl groups. The apparatus and methods uncouple or reduce the dependency between the monomer concentration and transfer agent concentration. The uncoupling in other embodiments may also be varied leading to multiple desirable effects.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of, and claims thebenefit of U.S. application Ser. No. 10/028,552, filed Dec. 19, 2001(now allowed), which is incorporated by reference herein.

FIELD OF THE INVENTION

[0002] This invention relates to an apparatus and processes for improvedpolymerization in tubular polymerization reactors, including those usingchain transfer agents and multiple monomer feeds spaced lengthwise alongthe tubular reactor, to provide high conversions of monomer intopolymer. The invention also relates to polymers made from such processesand apparatus, including those polymers having a low haze value, adensity over 0.92 g/cm³ and/or having terminal carbonyl groups.

BACKGROUND

[0003] Tubular polymerization reactor apparatus is used to makepolyethylene, mainly by free radical initiation. Initiators may beoxygen, peroxides and similar agents. Catalysts used for coordinationpolymerization may also, where appropriate, be used.

[0004] The highly exothermic polymerization reaction is performed in atubular reactor (“tube”) forming part of the apparatus under highreactor operating pressure (2000 bar to 3500 bar) under turbulent flow,at high temperatures (120° C. to 330° C.). Heat may be removed throughthe tube wall, which may be liquid cooled. Tubular reactors may haveoutputs, which vary from 50 kT to 400 kT per annum. Low cost productionrequires a high conversion of monomers to give as large an output ofcommercially desirable polymer types as is possible from giveninvestment.

[0005] Referring to FIG. 1, a tubular polymerization reactor 100 has atube 2 with a length typically from 200 to 1600 meters determined on thebasis of the desired heat removal capacity and a diameter of from 20 to100 mm determined on the basis of the desired capacity and the requiredturbulent flow.

[0006] A medium pressure, primary compressor 4, which may include anumber of compressor stages, not individually shown, is connected at itsintake side to a source of fresh ethylene supplied by a conduit 6, andrecycled ethylene from a recycle conduit 8 at a pressure of from 20 to70 bar. The primary compressor raises the pressure of the monomer on theoutlet side to a pressure of from 250 bar to 350 bar. A high pressure,secondary compressor 5, which may include a number of compressor stages,is connected at its intake side to the outlet side of the primarycompressor 4 and raises the pressure of the feed containing ethylenefurther to the reactor operating pressure as indicated above of from2000 to 3500 bar. The compressed pressurized monomer is then fed throughconduits 12, 14 to various monomer feed locations 3 spaced lengthwisealong tube 2.

[0007] Multiple free-radical initiator or catalyst injection positions 7are also spaced lengthwise of the tube 2 to cause the monomer to beconverted into polymer in at least two reaction zones formed inside thetube 2.

[0008] A mixture of polymer and unreacted monomer formed in the tube 2passes from tube outlet 16 to a separating and recycling part of thepolymerization apparatus. This part includes a high-pressure separator(HPS) 18, which receives the monomer-polymer mixture from the outlet ofthe tube 2. The HPS is connected to convey a mixture of polymer andmonomer produced, to a low-pressure separator (LPS) 20 for furthermonomer removal. The resulting molten polymer phase is passed from theLPS 20 to a polymer finishing section with an extruder 22. A volatilemonomer-rich phase comprising unreacted monomer separated in HPS 18,passes through a recycle conduit 24 at a pressure of approximately thatof the outlet of the primary compressor 4 through line 26 to join themonomer containing feed passing from the primary to the secondarycompressor 5. The volatile monomer rich phase including unreactedmonomer from the LPS 20 passes to a low pressure purge compressor 21,which may have a number of stages, at a pressure above that at theintake of the primary compressor to the intake of the primary compressor4.

[0009] At some location in the circuit a chain transfer agent is addedfor supply to the tube 2. Transfer agents are used to reduce themolecular weight, which can be expressed in a melt index (MI) value, andto narrow the molecular weight distribution (MWD).

[0010] A typical product range is shown in FIG. 2 and covers a meltindex (“MI”, I_(2.16)) of from 0.1 to 50 dg/min, a molecular weightdistribution (MWD) of from 5 to 50 and a haze of from 1 to 20. Haze isdetermined by ASTM D-1003; MI is determined by ASTM-1238 Condition E; Mwand Mn were measured by GPC (Gel Permeation Chromatography) on a Waters150 gel permeation chromatograph equipped with a differential refractiveindex (DRI) detector and a Chromatix KMX-6 on line light scatteringphotometer. The system was used at 135° C. with 1,2,4-trichlorobenzeneas the mobile phase. Shodex (Showa Denko America, Inc) polystyrene gelcolumns 802, 803, 804 and 805 were used. This technique is discussed in“Liquid Chromatography of Polymers and Related Materials III”, J. Cazeseditor, Marcel Dekker. 1981, p. 207, which is incorporated herein byreference. No corrections for column spreading were applied; however,data on generally accepted standards, e.g., National Bureau of StandardsPolyethylene 1484, and anionically produced hydrogenated polyisoprenes(an alternating ethylene-propylene copolymer) demonstrated that suchcorrections on Mw/Mn (=MWD) were less than 0.05 units. Mw/Mn wascalculated from elution times. The numerical analyses were performedusing the commercially available Beckman/CIS customized LALLS softwarein conjunction with the standard Gel Permeation package. Calculationsinvolved in the characterization of polymers by ¹³CNMR follow the workof F. A. Bovey in “Polymer Conformation and Configuration” AcademicPress, New York, 1969.

[0011] In practical use of the apparatus, product quality has to bebalanced with desired production economics. Higher conversion (givinglow energy and recycle costs) tends to lead to a broader MWD andsignificant branching which leads to high and unacceptable haze values.Low density polyethylene requires production of relatively many shortchain branches. Olefinically unsaturated comonomers are then used whichhave a low transfer coefficient (efficiency of transfer agents) andhence little chain length reducing activity. Examples are propylene orbutene-1. A high concentration of such comonomers is needed to achieve adesired melt index, restricting the productive capacity on a givenreactor. In some cases, certain areas of theoretically available MI,haze and density combinations cannot be produced at an acceptable cost.Particularly narrow molecular weight distribution (MWD), relatively highdensity polyethylenes generally cannot be made economically withsaturated alkane transfer agents (which do not incorporate in the chain)as they have a very low transfer constant, lower than the propylene andbutene-1 used for low density polyethylenes.

[0012] An initiator or catalyst injection position is associated witheach reaction zone. Injection of the initiator causes an exothermictemperature rise which is removed by a cooling at the zone anddownstream of that zone. The cooling is effected through the tube wall,optionally aided by a cooling liquid as a heat transfer medium and/or bya feed of cold monomer that is added downstream. Further, initiator maybe added downstream to form another reaction zone for convertingadditional monomer into polymer.

[0013] Generally speaking, in the prior art, transfer agents have beenadded so as to have roughly the same concentration of chain transferagent in each monomer feed. From an apparatus point of view, this can beachieved by mixing the transfer agent with the monomer fed before themonomer is compressed by the secondary compressor. The transfer agent isthen added equally along the length of the tube, although it may beconsumed unequally and so concentration variations along the tube mayresult.

[0014] In FIG. 1, a source 23 of transfer agent is connected to theintake of the primary compressor 4 and hence distributed, after passingthrough the secondary compressor 5, to the different monomer feeds 3spaced along the tube 2. The recycle stream 8 coming from the LPS 20 andthe purge compressor 21 is also passed to the intake of primarycompressor 4. The recycle from the HPS 18 which contains unconsumedtransfer agent is passed to the intake of the secondary compressor. Thusthe transfer agent and monomer form a single, common gas stream with thedesired concentration of transfer agent for compression in the secondarycompressor 5 and for supply to the various feed positions 3 along thetube 2.

[0015] Furthermore, by selecting a transfer agent which has a low chaintransfer activity, higher concentrations of transfer agent have to beused in the non-polymer gaseous fraction of the tube contents to achievea target MI. The low chain transfer activity contributes to the creationof small transfer agent concentrations along the length of the tubewhere the chain transfer agents also have a low reactivity ratio. Byusing unsaturated transfer agents with low chain transfer activity,branches are formed along the polymer backbone and the density of theresulting polymer is reduced. In such apparatus, mostly chain transferagents have been used having a chain transfer constant of less than0.03.

[0016] The recycle from the HPS and LPS still contain, for transferagents with a low reactivity ratio, a high level of transfer agent andthe amount that is added from the source 23 is low relative to thatpresent in the recycle 26 and 8.

[0017] It would be desirable to have methods and apparatus for transferagent addition and selection so as to increase the process efficiencywhile obtaining a satisfactory commercial product, or to produce moresatisfactory commercial products at prevailing process efficiencies.

SUMMARY

[0018] The present invention provides apparatus and methods whichadvantageously uncouple, or reduce the dependency, between the monomerconcentration and transfer agent concentration and permit theseconcentrations to be varied along the tube length.

[0019] In one embodiment the invention provides a tubular polymerizationreactor apparatus including a source of fresh monomer, first and secondcompressor stages for compressing monomer, a reactor tube, multiplefeeds spaced lengthwise along the reactor tube for supplying monomer tothe reactor, multiple free-radical or catalyst injection positionsspaced lengthwise along the reactor tube for causing monomer to beconverted into polymer inside the reactor, separators for receiving amonomer-polymer mixture from the reactor tube and separating the mixtureinto a volatile monomer-rich phase and molten polymerization phase,conduits for recycling the monomer-rich phase to the first and/or secondcompressor stages for recycling unreacted monomer to the reactor tube,and a source of transfer agent for modifying the molecular weight of thepolymer for compression and feeding to the reactor tube. Compressormeans is provided for compressing a transfer agent rich streamseparately from a transfer agent-poor monomer stream and means isprovided for feeding the compressed transfer agent rich stream to apolymerization reaction zone upstream of at least one reaction zonereceiving the transfer agent-poor stream.

[0020] In another embodiment the invention provides a tubularpolymerization reactor apparatus including a source of fresh monomer,first and second compressor stages for compressing monomer, a reactortube, multiple feeds spaced lengthwise along the reactor tube forsupplying monomer to the reactor, multiple free-radical or catalystinjection positions spaced lengthwise along the reactor tube for causingmonomer to be converted into polymer inside the reactor, separators forreceiving a monomer-polymer mixture from the reactor tube and separatingthe mixture into a volatile monomer-rich phase and molten polymerizationphase, conduits for recycling the monomer-rich phase to the first and/orsecond compressor stages for recycling unreacted monomer to the reactortube, and a source of transfer agent for modifying the molecular weightof the polymer for compression and feeding to the reactor tube.Compressor means is provided for compressing a transfer agent richstream separately from a transfer agent-poor monomer stream and means isprovided for feeding the compressed transfer agent rich stream to apolymerization reaction zone upstream of at least one reaction zonereceiving the transfer agent-poor stream containing 70 wt. % or less ofthe transfer agent relative to the transfer agent-rich stream.

[0021] In still another embodiment the invention provides a tubularpolymerization reactor apparatus including a source of fresh monomer,first and second compressor stages for compressing monomer, a reactortube, multiple feeds spaced lengthwise along the reactor tube forsupplying monomer to the reactor, multiple free-radical or catalystinjection positions spaced lengthwise along the reactor tube for causingmonomer to be converted into polymer inside the reactor, separators forreceiving a monomer-polymer mixture from the reactor tube and separatingthe mixture into a volatile monomer-rich phase and molten polymerizationphase, conduits for recycling the monomer-rich phase to the first and/orsecond compressor stages for recycling unreacted monomer to the reactortube, and a source of transfer agent for modifying the molecular weightof the polymer for compression and feeding to the reactor tube.Compressor means is provided for compressing a transfer agent richstream separately from a transfer agent-poor monomer stream and means isprovided for feeding the compressed transfer agent rich stream to apolymerization reaction zone upstream of at least one reaction zonereceiving the transfer agent-poor stream containing between 70 wt. % to30 wt. % of the transfer agent relative to the transfer agent-richstream.

[0022] In yet another embodiment the invention provides a tubularpolymerization reactor apparatus including a source of fresh monomer,first and second compressor stages for compressing monomer, a reactortube, multiple feeds spaced lengthwise along the reactor tube forsupplying monomer to the reactor, multiple free-radical or catalystinjection positions spaced lengthwise along the reactor tube for causingmonomer to be converted into polymer inside the reactor, separators forreceiving a monomer-polymer mixture from the reactor tube and separatingthe mixture into a volatile monomer-rich phase and molten polymerizationphase, conduits for recycling the monomer-rich phase to the first and/orsecond compressor stages for recycling unreacted monomer to the reactortube, and a source of transfer agent for modifying the molecular weightof the polymer for compression and feeding to the reactor tube.Compressor means is provided for compressing a transfer agent richstream separately from a transfer agent-poor monomer stream and means isprovided for feeding the compressed transfer agent rich stream to apolymerization reaction zone upstream of at least one reaction zonereceiving the transfer agent-poor stream containing less than 30 wt. %of the transfer agent relative to the transfer agent rich stream.

[0023] In still yet another embodiment the invention provides a processfor the polymerization of ethylene, wherein the process includescombining fresh monomer and recycled monomer; compressing the combinedmonomer; supplying the monomer using multiple feeds to multiple reactionzones in a tubular reactor for polymerization by a free radicalinitiator to form a monomer-polymer mixture; separating the mixture intoa volatile monomer-rich phase and molten polymer-rich phase; recyclingfor compression the monomer-rich phase and supplying the monomer-richphase to the reactor; and introducing transfer agent into the reactor tomodify the molecular weight of the polymer, and further wherein thetransfer agent includes a chain terminating transfer agent having atransfer coefficient of greater than 0.01, the transfer agent being in atransfer agent rich stream separately from a transfer agent-poor monomerstream of the transfer agent, to a polymerization reaction zone upstreamof at least one downstream reaction zone receiving the transferagent-poor stream so as to achieve a depletion in the concentration ofthe transfer agent in the downstream reaction zone.

[0024] In yet still another embodiment the invention provides a processfor the polymerization of ethylene, wherein the process includescombining fresh monomer and recycled monomer; compressing the combinedmonomer; supplying the monomer using multiple feeds to multiple reactionzones in a tubular reactor for polymerization by a free radicalinitiator to form a monomer-polymer mixture; separating the mixture intoa volatile monomer-rich phase and molten polymer-rich phase; recyclingfor compression the monomer-rich phase and supplying the monomer-richphase to the reactor; and introducing transfer agent into the reactor tomodify the molecular weight of the polymer, and further wherein thetransfer agent includes a chain terminating transfer agent having atransfer coefficient of greater than 0.01, the transfer agent being in atransfer agent rich stream separately from a transfer agent-poor monomerstream of the transfer agent, to a polymerization reaction zone upstreamof at least one downstream reaction zone receiving the transferagent-poor stream having 70 wt. % or less of the transfer agent relativeto the transfer agent rich stream so as to achieve a depletion in theconcentration of the transfer agent in the downstream reaction zone.

[0025] An additional embodiment of the invention is a process for thepolymerization of ethylene, wherein the process includes combining freshmonomer and recycled monomer; compressing the combined monomer;supplying the monomer using multiple feeds to multiple reaction zones ina tubular reactor for polymerization by a free radical initiator to forma monomer-polymer mixture; separating the mixture into a volatilemonomer-rich phase and molten polymer-rich phase; recycling forcompression the monomer-rich phase and supplying the monomer-rich phaseto the reactor; and introducing transfer agent into the reactor tomodify the molecular weight of the polymer, and further wherein thetransfer agent includes a chain terminating transfer agent having atransfer coefficient of greater than 0.01, the transfer agent being in atransfer agent rich stream separately from a transfer agent-poor monomerstream of the transfer agent, to a polymerization reaction zone upstreamof at least one downstream reaction zone receiving the transferagent-poor stream having between 70 wt. % and 30 wt. % of the transferagent relative to the transfer agent rich-stream so as to achieve adepletion in the concentration of the transfer agent in the downstreamreaction zone.

[0026] Even another embodiment the invention provides a process for thepolymerization of ethylene, wherein the process includes combining freshmonomer and recycled monomer; compressing the combined monomer;supplying the monomer using multiple feeds to multiple reaction zones ina tubular reactor for polymerization by a free radical initiator to forma monomer-polymer mixture; separating the mixture into a volatilemonomer-rich phase and molten polymer-rich phase; recycling forcompression the monomer-rich phase and supplying the monomer-rich phaseto the reactor; and introducing transfer agent into the reactor tomodify the molecular weight of the polymer, and further wherein thetransfer agent includes a chain terminating transfer agent having atransfer coefficient of greater than 0.01, the transfer agent being in atransfer agent rich stream separately from a transfer agent-poor monomerstream of the transfer agent, to a polymerization reaction zone upstreamof at least one downstream reaction zone receiving the transferagent-poor stream having less than 30 wt. % of the transfer agentrelative to the transfer agent rich stream so as to achieve a depletionin the concentration of the transfer agent in the downstream reactionzone.

BRIEF DESCRIPTION OF DRAWINGS

[0027] These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings, where:

[0028]FIG. 1 shows a tubular polymerization reactor.

[0029]FIG. 2 shows a typical polyethylene product range and covers amelt index (MI) of from 0.1 to 50, a molecular weight distribution (MWD)of from 5 to 50 and a haze of from 1 to 20.

[0030]FIG. 3 shows one embodiment of a reactor apparatus of theinvention.

DETAILED DESCRIPTION

[0031] In one embodiment, and referring now to FIG. 3, the presentinvention provides a tubular polymerization reactor apparatus 200having:

[0032] a source of fresh monomer 6, unrecycled, generally not containingtransfer agent;

[0033] medium and high pressure compressors 4 and 5, respectively, forcompressing monomer;

[0034] a reactor tube 2;

[0035] multiple monomer feed 3, spaced lengthwise along the reactor tube2 for supplying monomer to the tube;

[0036] multiple free-radical or catalyst injection positions 7 spacedlengthwise along the tube 2 for causing monomer to be converted intopolymer inside the tube in a reaction zone (not shown);

[0037] separators 18 and 20 for receiving a monomer-polymer mixture fromthe reactor tube 2 and separating the mixture into a volatilemonomer-rich phase and molten, polymer-rich phase;

[0038] conduits 8 and 26 for recycling the monomer-rich phase forcompression and for recycling unreacted monomer or monomers to thetubular reactor; and

[0039] a source of transfer agent 30 for modifying the molecular weightof the polymer including means 32 for compressing the transfer agent andfeeding it to the tubular reactor via one or more transfer agent feeds34 separately from the monomer feed(s) 3.

[0040] In the conventional apparatus described above with reference toFIG. 1, transfer agent and monomer were mixed before a final commoncompression step and so supplied at equal transfer agent/monomer ratiosat the different feeds 3. By contrast, in embodiments of the invention,compressor means 32 is provided for compressing a transfer agent richgas stream 30 separately from a transfer agent-poor gas stream 12, and atransfer agent feeds 34 are provided for feeding the compressed transferagent-rich stream to a polymerization reaction zone upstream of at leastone reaction zone receiving the transfer agent-poor stream. As a resultless transfer agent is supplied towards the downstream end of thetubular reactor, i.e., the part of the reactor containing one or morereaction zones downstream of one or more reaction zones located towardsthe upstream end of the reactor than is supplied towards those upstreamreaction zone or zones.

[0041] Examples of chain transfer agents include tetramethylsilane,cyclopropane, sulfur hexafluoride, methane, t-butanol, perfluoropropane,deuterobenzene, ethane, ethylene oxide, 2,2-dimethylpropane, benzene,dimethyl sulfoxide, vinyl methyl ether, methanol, propane,2-methyl-3-buten-2-ol, methyl acetate, t-butyl acetate, methyl formate,ethyl acetate, butane, triphenylphosphine, methylamine, methyl benzoate,ethyl benzoate, N,N-diisopropylacetamide, 2,2,4-trimethylpentane,n-hexane, isobutane, dimethoxymethane, ethanol, n-heptane, n-butylacetate, cyclohexane, methylcyclohexane, 1,2-dichlorethane,acetronitrile, N-ethylacetamide, propylene, n-decane,N,N-diethylacetamide, cyclopentane, acetic anhydride, n-tridecane,n-butyl benzoate, isopropanol, toluene, hydrogen, acetone,4,4-dimethylpentene-1, trimethylamine, N,N-dimethylacetamide,isobutylene, n-butyl isocyanate, methyl butyrate, n-butylamine,N,N-dimethylformamide, diethyl sulfide, diisobutylene, tetrahydrofuran,4-methylpentene-1, p-xylene, p-dioxane, trimethylamine, butene-2,1-bromo-2-chlorethane, octene-1, 2-methylbutene-2, cumene, butene-1,methyl vinyl sulfide, n-butyronitrile, 2-methylbutene-1, ethylbenzene,n-hexadecene, 2-butanone, n-butyl isothiocyanate, methyl3-cyanopropionate, tri-n-butylamine, 3-methyl-2-butanone,isobutyronitrile, di-n-butylamine, methyl chloroacetate,3-methylbutene-1, 1,2-dibromoethane, dimethylamine, benzaldehyde,chloroform, 2-ethylhexene-1, propionaldehyde, 1,4 dichlorobutene-2,tri-n-butylphosphine, dimethylphosphine, methyl cyanoacetate, carbontetrachloride, bromotrichloromethane, di-n-butylphosphine, acetaldehyde,and phosphine.

[0042] For further details of transfer agents, see Advances In PolymerScience, Vol. 7, p. 386-448 (1970). Table 7 therein ranks severaltransfer agents in order of the chain transfer constant determined underset conditions. The tendency to copolymerize is indicated by thereactivity, also determined under set conditions.

[0043] Typical monomers include: vinyl ethers such as vinyl methylether, vinyl n-butyl ether, vinyl phenyl ether, vinyl beta-hydroxy-ethylether, and vinyl dimethylamino-ethyl ether; olefins such as ethylene,propylene, butene-1, cis-butene-2, trans-butene-2, isobutylene,3,3,-dimethylbutene-1, 4-methylpentene-1, octene-1, and styrene; vinyltype-esters such as vinyl acetate, vinyl butyrate, vinyl pivalate, andvinylene carbonate; haloolefins such as vinyl fluoride, vinylidenefluoride, tetrafluoroethylene, vinyl chloride, vinylidene chloride,tetrachloroethylene, and chlorotrifluoroethylene; acrylic-type esterssuch as methyl acrylate, ethyl acrylate, n-butyl acrylate, t-butylacrylate, 2-ethylhexyl acrylate, alpha-cyanoisopropyl acrylate,beta-cyanoethyl acrylate, o-(3-phenylpropan-1,3,-dionyl)phenyl acrylate,methyl methacrylate, n-butyl methacrylate, t-butyl methacrylate,cyclohexyl methacrylate, 2-ethylhexyl methacrylate, methyl methacrylate,glycidyl methacrylate, betahydroxethyl methacrylate, beta-hydroxpropylmethacrylate, 3-hydroxy-4-carbomethoxy-phenyl methacrylate,N,N-dimethylaminoethyl methacrylate, t-butylaminoethyl methacrylate,2-(1-aziridinyl)ethyl methacrylate, diethyl fumarate, diethyl maleate,and methyl crotonate; other acrylic-type derivatives such as acrylicacid, methacrylic acid, crotonic acid, maleic acid, methyl hydroxy,maleate, itaconic acid, acrylonitrile, fumaronitrile,N,N-dimethylacrylamide, N-isopropylacrylamide, N-t-butylacrylamide,N-phenylacrylamide, diacetone acrylamide, methacrylamide,N-phenylmethacrylamide, N-ethylmaleimide, and maleic anhydride; andother compounds such as allyl alcohol, vinyltrimethylsilane,vinyltriethoxysilane, N-vinylcarbazole, N-vinyl-N-methylacetamide,vinyldibutylphosphine oxide, vinyldiphenylphosphine oxide,bis-(2-chloroethyl) vinylphosphonate and vinyl methyl sulfide. Themonomers/transfer agents are ranked in reactivity ratio order in Table10. See id.

[0044] The difference in the concentration in mol % of transfer agentsupplied to the successive reaction zones may be in excess of 20 %. Asimple constructional option for implementing a difference of such amagnitude is providing conduits for compressing some or all of thetransfer agent which are separate from other conduits for compressingmonomer. Therefore compressor means 32 may include a further compressorstage for an initial compression of the transfer agent obtained from thesource of transfer agent 30, and a section of the second compressorstage connected to the outlet of the further compressor stage forraising the gas stream containing transfer agent to a pressure suitablefor supplying to the reactor, the section being optionally usedadditionally for compressing part of the monomer obtained from theoutlet of the first compressor stage, the transfer agent being passedthrough conduits segregated from another section of the secondcompressor stage which is used for compressing the transfer agent-poormonomer stream. In this manner fresh transfer agent is concentrated, tohave effect in the upstream reaction zone.

[0045] The transfer agent rich stream may be connected to be introducedupstream of all reaction zones receiving a transfer agent-poor stream.This may magnify the effect and benefit of embodiments of the invention.In another embodiment, where three or more reaction zones are used, thetransfer agent rich stream may be supplied to a reaction zoneintermediate two reaction zones receiving a transfer agent poor-stream.This may tend to dilute the benefit of upstream transfer agentinjection.

[0046] An added modification in another embodiment is to furtheruncouple the volatile part of the mixture from the tubular reactoroutlet from the general monomer feed and to not distribute the residualtransfer agent present therein to the feed streams used for thedownstream reaction zones. The monomer-rich recycled stream from thecombined separators is connected to be supplied to an extent exceedingfrom 75% by volume of the total recycle stream mass to one or morereaction zones upstream of at least one downstream reaction zone. Theremaining part of the recycle stream mass can be combined with themonomer feed supplied to the medium pressure compressor intake fordistribution to all other feed positions. The effect can be optimized byproviding a recycled monomer-rich feed connected to be supplied to anextent of from 75 to 100% of its volume to a first reaction zone locatedfurthest upstream of all other reaction zones spaced lengthwise alongthe tubular reactor.

[0047] Construction of an apparatus for providing such an arrangementresults where the transfer agent-rich stream is connected to be passedfrom the further compressor stage and combined with the recycledmonomer-rich stream for compression in the segregated section of thesecond compressor stage.

[0048] While the monomer can be any molecule or molecules capable ofaddition polymerization by either a free-radical mechanism orcoordination catalytic mechanism, the predominant monomer may beethylene. Other monomers which incorporate less easily and may havetransfer-activity and a molecular weight limiting effect (and indeedcan, for some purposes, be regarded as incorporating transfer agents)include: vinyl acetate, ethyl acrylate, methyl acrylate, butyl acrylate,and the like. Most commonly ethylene is used at a mole concentration ofat least 90%, or 96%, or 98%, the percentages being based on the totalweight of all monomer and transfer agent present.

[0049] While, in theory, Ziegler-Natta catalysts can be used such asTiCl₃ based catalysts with an aluminum alkyl activator, or metallocenecatalysts with an alumoxane or non-coordinating anion activator, orusing a free-radical initiator, generally initiators can be selectedfrom the list given in Advances of Polymer Science op cit.

[0050] Preferably, the transfer agents have the properties:

[0051] they possess high transfer activity which enables them to reducethe melt index (MI) at relatively low concentrations of transfer agents.A review of the transfer agents in Advances of Polymer Science op citwill show the suitable candidates. Particularly suited are methyl vinylsulfide, and n-butyronitrile; and

[0052] they incorporate and have a high reactivity ratio so theconcentration will deplete, in the absence of further additiondownstream, as the reaction mixture passes downstream in the tubethrough the successive reaction zones, with the proviso that suchtransfer agents to do not lower the density and incorporate at the endsof the polymer chain without creating branches.

[0053] Other transfer agents may be present in addition, to formshort-chain branches to the extent that the desired conversion enhancingeffect is not negated.

[0054] Operating conditions for tubular reactors are in general wellknown, but the amount of monomer vs. transfer agent fed canadvantageously be biased to achieve a high transfer agent concentrationin the upstream feed or feeds.

[0055] The addition of the free-radical initiator to a monomer mixturecauses generally the formation of a reaction zone in which monomer isconverted exothermally and the resulting temperature rise is controlledby cooling and addition of further monomer to the downstream end of thereaction zone. Thus, temperature and reagent concentration can varyalong the length of the tubular reactor.

[0056] Embodiments of the invention generally provide, for this aspect,a process for polymerization of ethylene which includes combining freshmonomer and recycled monomer and compressing the combined recycled andfresh monomer and supplying the monomer using multiple feeds to multiplereaction zones in a tubular reactor for polymerization by a free radicalinitiator to form a monomer-polymer mixture; and separating the mixtureinto a volatile monomer-rich phase and molten polymer-rich phase withthe monomer-rich phase being recycled for compression and supplying tothe reactor, the process further including introducing transfer agentinto the reactor to modify the molecular weight of the polymer.

[0057] In such embodiments, the transfer agent includes a chainterminating transfer agent of a carbonyl containing linear compoundincorporating predominantly in a position close to the end of thepolymer chain, having a transfer coefficient of greater than 0.01, thetransfer agent being in a transfer agent rich stream separately from atransfer agent-poor monomer stream to a polymerization reaction zoneupstream of at least one reaction zone receiving the transfer agent-poorstream.

[0058] As indicated above, the chain terminating transfer agent richstream is introduced preferably upstream of all reaction zones receivinga transfer agent-poor stream. Similarly the monomer-rich recycled streamfrom at least one of the separators is advantageously supplied to anextent exceeding 75% by volume to one or more reaction zones upstream ofat least one downstream reaction zone. Suitably a recycled monomer-richfeed is supplied to an extent of from 75 to 100% of its volume to areaction zone upstream of all other reaction zones spaced lengthwisealong the tubular reactor.

[0059] A particular chain terminating transfer agent contemplated maypredominantly be propionaldehyde. Its effects may be focused on one ormore reaction zones. Propionaldehyde may cause a linear polymer to beformed, which can act as the platform for added polymerization indownstream polymerization reaction zones and higher conversion therein,without materially upsetting product quality. The chain terminatingtransfer agent may be added in such amounts in such a way that theconcentration of chain terminating transfer agent at the outlet of thetube is at least 50% less than that at the uppermost upstream reactionzone.

[0060] While in a conventional process using an almost homogeneoustransfer agent distribution, the process is run so as to convert as muchof the polymer in the first reaction zone as possible, it has beensurprisingly found that advantageous operation results by increasing theproportion of transfer agent further by reducing the amount of monomersupplied upstream, particularly the first upstream reaction zone. Themol % of monomer fed in the stream supplied to the reaction zone locatedfurthest upstream may be less than that at least one reaction zonedownstream thereof.

[0061] By using a chain terminating transfer agent which gives by andlarge a linear polyethylene chain backbone, the process may not have tobe manipulated to obtain desired levels of branching and branching maybe provided by a transfer agent selected for that purpose. Such chaintransfer agents may incorporate poorly such as propylene or butene-1 butpreferably are selected to incorporate well, such as methylmethacrylate, as noted supra. In this way, the monomer rich part of therecycle contains relatively low levels of chain forming transfer agentsfor introduction in the upstream reaction zones.

[0062] The chain branch forming transfer agent may be fed predominantlyto reaction zone or zones downstream of an upstream reaction zone.

[0063] Although the present invention has been described in considerabledetail with reference to certain preferred versions thereof, otherversions are possible. For example, while polyethylenes are discussed,other polyolefins are contemplated. Therefore, the spirit and scope ofthe appended claims should not be limited to the description of thepreferred versions contained herein, which include, without limitation,the following more preferred embodiments: a tubular polymerizationreactor apparatus comprising: (a) a source of fresh monomer; (b) firstand second compressor stages for compressing monomer; (c) a reactortube; (d) multiple feeds, spaced lengthwise along the reactor tube forsupplying monomer to the reactor; (e) multiple free-radical or catalystinjection positions spaced lengthwise along the tubular reactor forcausing monomer to be converted into polymer inside the tubular reactor;(f) separators for receiving a monomer-polymer mixture from the reactortube and separating said mixture into a volatile monomer-rich phase andmolten polymerization phase; (g) conduits for recycling the monomer-richphase to the first and/or second compressor stages for recyclingunreacted monomer to the reactor tube; and (h) a source of transferagent for modifying the molecular weight of the polymer for compressionand feeding to the reactor tube; wherein compressor means is providedfor compressing a transfer agent rich stream separately from a transferagent-poor monomer stream and means is provided for feeding thecompressed transfer agent rich stream to a polymerization reaction zoneupstream of at least one reaction zone receiving the transfer agent-poorstream; and also the following even more preferred embodiments: thereactor apparatus wherein said transfer agent-poor stream contains lessthan 30 wt. % of the transfer agent relative to the transfer agent richstream; the reactor apparatus wherein said transfer agent-poor streamcontains 70 wt. % or less of the transfer agent relative to the transferagent-rich stream; the reactor apparatus wherein said transferagent-poor stream contains between 70 wt. % and 30 wt. % of the transferagent relative to the transfer agent-rich stream; and any of theaforementioned more preferred or even more preferred embodiments withthe following additionally more preferred embodiments, alone or incombination as would be apparent to one of skill in the art inpossession of the present disclosure: the reactor apparatus whereincompressor means comprises a further compressor stage for an initialcompression of the transfer agent obtained from the source of transferagent, and a section of the second compressor stage, connected to theoutlet of the further compressor stage for raising the gas streamcontaining transfer agent to a pressure suitable for supplying to thereactor, said section being optionally used additionally for compressingpart of the monomer obtained from the outlet of the first compressorstage, said transfer agent being passed through conduits segregated fromanother section of the second compressor stage which is used forcompressing the transfer agent-poor monomer stream; the reactorapparatus wherein the transfer agent rich stream is connected to beintroduced upstream of all reaction zones receiving a transferagent-poor stream, particularly wherein the monomer-rich recycled streamfrom at least one of the separators is connected to be supplied to anextent exceeding 75% by volume to one or more reaction zones upstream ofat least one downstream reaction zone, or even more particularly whereina recycled monomer-rich feed is connected to be supplied to an extent offrom 75 to 100% of its volume to a reaction zone upstream of all otherreaction zones spaced lengthwise along the tubular reactor; the reactorapparatus wherein the transfer agent is connected to be passed from thefurther compressor stage and combined with the recycled monomer-richstream for compression in the segregated section of the secondcompressor stage, particularly wherein a pair of sources of transferagents are arranged for connection to different reaction zones; andanother more preferred embodiment which is a process for thepolymerization of ethylene comprising: (a) combining fresh monomer andrecycled monomer and compressing the combined monomer and supplying themonomer using multiple feeds to multiple reaction zones in a tubularreactor for polymerization by a free radical initiator to form amonomer-polymer mixture; (b) separating the mixture into a volatilemonomer-rich phase and molten polymer-rich phase; (c) recycling forcompression said monomer-rich phase and supplying said monomer-richphase to the reactor; and (d) introducing transfer agent into thereactor to modify the molecular weight of the polymer; wherein thetransfer agent comprises a chain terminating transfer agent having atransfer coefficient of greater than 0.01, said transfer agent being ina transfer agent rich stream separately from a transfer agent-poormonomer stream of the transfer agent, to a polymerization reaction zoneupstream of at least one downstream reaction zone receiving the transferagent-poor stream so as to achieve a depletion in the concentration ofthe transfer agent in said at least one downstream reaction zone; andeven more preferred embodiments which are said process wherein saidtransfer agent-poor stream comprises less than 30 wt. % of the transferagent relative to the transfer agent-rich stream; wherein said transferagent-poor stream comprises 70 wt. % or less of the transfer agentrelative to the transfer agent-rich stream; said process wherein saidtransfer agent-poor stream comprises between 70 wt. % and 30 wt. % ofthe transfer agent relative to the transfer agent-rich stream; or any ofthe aforementioned more preferred or even more preferred processes,alone or in combination as would be apparent to one of ordinary skill inthe art in possession of the present disclosure, of the following: saidprocess wherein the transfer agent-rich stream is introduced upstream ofall reaction zones receiving a transfer agent-poor stream; said processwherein the monomer-rich recycled stream from at least one of theseparators is supplied to an extent exceeding 75% by volume to one ormore reaction zones upstream of at least one downstream reaction zone,and more particularly wherein a recycled monomer-rich feed is suppliedto an extent of from 75 to 100% of its volume to a reaction zoneupstream of all other reaction zones spaced lengthwise along the tubularreactor, even more particularly wherein the transfer agent supplied atan upstream feed comprises propionaldehyde and/or wherein theconcentration of transfer agent at the outlet of the tubular reactor isat least 50% less than the concentration transfer agent in the uppermostupstream reaction zone; wherein the transfer agent is selected from thegroup consisting of propionaldehyde, methyl vinyl sulfide,n-butyronitrile, propylene, butene-1, 4-methylpentane-1, isobutylene,diisobutylene, acetaldehyde, and mixtures thereof.

What is claimed is:
 1. A tubular polymerization reactor apparatuscomprising: (a) a source of fresh monomer; (b) first and secondcompressor stages for compressing monomer; (c) a reactor tube; (d)multiple feeds, spaced lengthwise along the reactor tube for supplyingmonomer to the reactor; (e) multiple free-radical or catalyst injectionpositions spaced lengthwise along the tubular reactor for causingmonomer to be converted into polymer inside the tubular reactor; (f)separators for receiving a monomer-polymer mixture from the reactor tubeand separating said mixture into a volatile monomer-rich phase andmolten polymerization phase; (g) conduits for recycling the monomer-richphase to the first and/or second compressor stages for recyclingunreacted monomer to the reactor tube; and (h) a source of transferagent for modifying the molecular weight of the polymer for compressionand feeding to the reactor tube; wherein compressor means is providedfor compressing a transfer agent rich stream separately from a transferagent-poor monomer stream and means is provided for feeding thecompressed transfer agent rich stream to a polymerization reaction zoneupstream of at least one reaction zone receiving the transfer agent-poorstream.
 2. The reactor apparatus of claim 1, wherein said transferagent-poor stream contains less than 30 wt. % of the transfer agentrelative to the transfer agent rich stream.
 3. The reactor apparatus ofclaim 1, wherein said transfer agent-poor stream contains 70 wt. % orless of the transfer agent relative to the transfer agent-rich stream.4. The reactor apparatus of claim 1, wherein said transfer agent-poorstream contains between 70 wt. % and 30 wt. % of the transfer agentrelative to the transfer agent-rich stream.
 5. The reactor apparatus ofclaim 1, wherein compressor means comprises a further compressor stagefor an initial compression of the transfer agent obtained from thesource of transfer agent, and a section of the second compressor stage,connected to the outlet of the further compressor stage for raising thegas stream containing transfer agent to a pressure suitable forsupplying to the reactor, said section being optionally usedadditionally for compressing part of the monomer obtained from theoutlet of the first compressor stage, said transfer agent being passedthrough conduits segregated from another section of the secondcompressor stage which is used for compressing the transfer agent-poormonomer stream.
 6. The reactor apparatus of claim 1, wherein thetransfer agent rich stream is connected to be introduced upstream of allreaction zones receiving a transfer agent-poor stream.
 7. The reactorapparatus of claim 6, wherein the monomer-rich recycled stream from atleast one of the separators is connected to be supplied to an extentexceeding 75% by volume to one or more reaction zones upstream of atleast one downstream reaction zone.
 8. The reactor apparatus of claim 7,wherein a recycled monomer-rich feed is connected to be supplied to anextent of from 75 to 100% of its volume to a reaction zone upstream ofall other reaction zones spaced lengthwise along the tubular reactor. 9.The reactor apparatus of claim 5, wherein the transfer agent isconnected to be passed from the further compressor stage and combinedwith the recycled monomer-rich stream for compression in the segregatedsection of the second compressor stage.
 10. The reactor apparatus ofclaim 9, wherein a pair of sources of transfer agents are arranged forconnection to different reaction zones.
 11. A process for thepolymerization of ethylene comprising: (a) combining fresh monomer andrecycled monomer and compressing the combined monomer and supplying themonomer using multiple feeds to multiple reaction zones in a tubularreactor for polymerization by a free radical initiator to form amonomer-polymer mixture; (b) separating the mixture into a volatilemonomer-rich phase and molten polymer-rich phase; (c) recycling forcompression said monomer-rich phase and supplying said monomer-richphase to the reactor; and (d) introducing transfer agent into thereactor to modify the molecular weight of the polymer; wherein thetransfer agent comprises a chain terminating transfer agent having atransfer coefficient of greater than 0.01, said transfer agent being ina transfer agent rich stream separately from a transfer agent-poormonomer stream of the transfer agent, to a polymerization reaction zoneupstream of at least one downstream reaction zone receiving the transferagent-poor stream so as to achieve a depletion in the concentration ofthe transfer agent in said at least one downstream reaction zone. 12.The process of claim 11, wherein said transfer agent-poor streamcomprises less than 30 wt. % of the transfer agent relative to thetransfer agent-rich stream.
 13. The process of claim 11, wherein saidtransfer agent-poor stream comprises 70 wt. % or less of the transferagent relative to the transfer agent-rich stream.
 14. The process ofclaim 11, wherein said transfer agent-poor stream comprises between 70wt. % and 30 wt. % of the transfer agent relative to the transferagent-rich stream.
 15. The process of claim 11, wherein the transferagent-rich stream is introduced upstream of all reaction zones receivinga transfer agent-poor stream.
 16. The process of claim 15, wherein themonomer-rich recycled stream from at least one of the separators issupplied to an extent exceeding 75% by volume to one or more reactionzones upstream of at least one downstream reaction zone.
 17. The processof claim 16, wherein a recycled monomer-rich feed is supplied to anextent of from 75 to 100% of its volume to a reaction zone upstream ofall other reaction zones spaced lengthwise along the tubular reactor.18. The process of claim 17, wherein the transfer agent supplied at anupstream feed comprises propionaldehyde.
 19. The process of claim 17,wherein the concentration of transfer agent at the outlet of the tubularreactor is at least 50% less than the concentration transfer agent inthe uppermost upstream reaction zone.
 20. The process of claim 11,wherein the transfer agent is selected from the group consisting ofpropionaldehyde, methyl vinyl sulfide, n-butyronitrile, propylene,butene-1, 4-methylpentane-1, isobutylene, diisobutylene, acetaldehyde,and mixtures thereof.